1. Field of the Invention
The present invention relates to a semiconductor epitaxial structure in which a plurality of semiconductor epitaxial films are grown on a substrate, and a semiconductor light-emitting device incorporating this epitaxial structure.
2. Description of the Related Art
Light-emitting devices comprising aluminum gallium arsenide (AlGaAs) compound semiconductor epitaxial layers are well known. A semiconductor light-emitting device having a p-type AlxGa1-xAs active layer (x=0.35) sandwiched between a p-type AlxGa1-xAs cladding layer (x=0.65) and an n-type AlxGa1-xAs cladding layer (x=0.65) is described by Y. Okuno in Hakko Diodo (Light-Emitting Diodes), published by Sangyo Tosho (1993). A semiconductor light-emitting device having a non-doped AlxGa1-xAs active layer (x=0.2) sandwiched between a p-type AlxGa1-xAs cladding layer (x=0.4) and an n-type AlxGa1-xAs cladding layer (x=0.4) is disclosed in FIG. 4 of Japanese Unexamined Patent Application Publication No. 11-340501 (1999).
In the first of these conventional devices, a pn junction is formed at the interface between the p-type active layer and the n-type cladding layer. When a forward voltage is applied, electrons in the conduction band are injected as minority carriers from the n-type cladding layer into the p-type active layer. If the energy barrier formed at the interface between the p-type active layer and the p-type cladding layer is sufficiently high, the injected electrons cannot diffuse into the p-type cladding layer and are confined to the p-type active layer. In the valence band, if the energy barrier between the p-type active layer and the n-type cladding layer is sufficiently high, holes are unable to move into the n-type cladding layer. The holes, however, are majority carriers in the p-type region comprising the p-type cladding layer and the p-type active layer, where they can be regarded as being distributed evenly. A consequent problem is that, although the electron-hole recombination rate is enhanced by a heightened concentration of injected electrons in the active layer, it is not enhanced by a heightened concentration of injected holes.
In the second conventional device mentioned above, if the flow of forward current is increased to increase the optical output, the number and density of carriers injected into the active layer increases, but if the density of injected carriers in the active layer becomes extremely high, carriers overflow from the active layer into the cladding layers, and the optical output does not continue to increase in proportion to the current. Furthermore, if the majority carrier density (impurity doping carrier concentration) is very low in the active layer, the injected minority carrier density is much higher than the majority carrier density in the active layer. An active layer not doped with an impurity can provide a very low majority carrier density. In such cases, the emitted light power or power density (emitted light power per unit area) is mainly proportional to the square of the applied current density (applied current per unit area). The operating characteristic (optical output v. current) therefore has poor linearity in the high current injection region.
If the thickness of the active layer in the second conventional device above is increased to reduce carrier overflow, or if the energy barrier at the active-cladding interfaces is increased, the p-to-n resistance of the hetero-epitaxial structure formed by the p-type cladding layer, non-doped active layer, and n-type cladding layer increases, increasing the power consumption of the device. This is problematic, because a semiconductor light-emitting device is frequently required to have a specified resistance and operate without consuming more than a specified amount of power.